1. Field of the Invention
The present invention relates to a contact probe for a testing head effective to test a plurality of semiconductor-integrated electronic devices comprising a plurality of so-called contact pads.
The invention particularly, but not exclusively, relates to a testing head having vertical probes for testing semiconductor-integrated electronic devices and the following description is made with reference to this field of application for convenience of illustration only.
2. Description of the Related Art
As it is well known, a testing head is essentially a device effective to electrically interconnect a plurality of contact pads of a semiconductor-integrated electronic device with corresponding channels of a testing machine which performs the test thereof.
The test carried out on integrated electronic devices is effective to detect and isolate devices already faulty during the manufacturing step. Normally, testing heads are thus used for the electric test of the semiconductor-integrated electronic devices, or integrated on a silicon wafer, before cutting and assembling the same inside a package for containing chips.
The connection between the testing apparatus and the contact pads of the tested integrated electronic device being tested is carried out by using a plurality of contact elements or probes.
In particular, these contact probes are normally made of wires of a special alloy having good electrical and mechanical properties.
The good connection between the probes and the contact pads is also ensured by the pressure of each contact probe on the respective contact pad.
A testing head having vertical probes comprises at least a pair of dies or plate-like parallel holders placed at a certain distance from one another in order to leave an air gap as well as a plurality of contact probes.
Each plate-like holder, called die in the technical field here considered and hereafter in the description, is provided with a respective plurality of through-going guide holes, each hole in one of the dies corresponding to a hole in the other die, wherein a respective contact probe is slidingly engaged and guided. In particular, the movable contact probes elastically flex inside the air gap between the two dies.
Testing heads of this type are commonly called “vertical probes”.
As schematically shown in FIG. 1, a testing head 1 of the known type comprises at least an upper die 2 and a lower die 3, having respective upper 4 and lower 5 through-going guide holes wherein at least one contact probe 6 is slidingly engaged.
The contact probe 6 has a contact end or tip 7. In particular, the contact tip 7 is in mechanical contact with a contact pad 8 of an integrated electronic device to be tested, performing at the same time the electrical contact between said device and a testing apparatus (not shown) of which this testing head is a terminal element.
The upper 2 and lower 3 dies are suitably spaced from an air gap 9 which allows the deformation or inclination of the contact probes 6 during the normal operation of the testing head, i.e. when this testing head comes into contact with the integrated electronic device to be tested. Moreover, the upper 4 and lower 5 guide holes are sized in order to guide the contact probe 6.
FIG. 1 schematically shows a testing head 1 having loose-fitting probes associated with a micro-contact strip or space transformer, globally indicated with 10.
In this case, the contact probes 6 have a further contact tip towards a plurality of contact pads 11 of the space transformer 10, the good electrical contact between probes and space transformer 10 being ensured in a similar way to the contact with the integrated electronic device to be tested through the pressure of the probes 6 on the contact pads 11 of the space transformer 10.
A very felt problem in the specific technical field is that of the reduction of the distance which separates the contact pads 8 of the device being tested, this distance being known in the field as “pitch”.
In particular, the technological development and the miniaturization of the chips requires the continuous reduction of the pitch of the integrated electronic device to be tested, and thus the distance between two adjacent probes 6 of the testing head which performs the test thereof.
The minimum pitch depends on the geometrical conformation and on the sizes of the probes 6, according to the relation:Pitch min=E+2A min+W min
being A min=(F−E)/2
Where, as illustrated in FIG. 2 which shows a section view of a part of the testing head 1 according to the prior art:
Pitchmin is the minimum pitch, i.e. the minimum distance between the centers of two adjacent contact pads 8 of the integrated electronic device to be tested;
E is the value of the cross section axis of the probe 6.
Wmin is the minimum value of the wall thickness between a guide hole 4 and 5 and the subsequent, such as to guarantee a certain mechanical strength to the structure of the testing head 1; and
F is the value of the cross section axis of the guide hole 4.
In the current vertical technologies, normally using probes having circular cross section, a reduction of the pitch value is obtained by reducing the size of the probe 6, in particular their minimum axis E (corresponding to the minimum diameter in the case of probes having circular section), being the other factors of the preceding relation practically fixed by the technological limits of realization of the testing heads.
This solution collides with the need of using probes 6 having sufficiently high section, able to guarantee a strength of the contact carried out by means of these probes, still miniaturizing the contacts.
Moreover, most integrated devices have contact pads arranged on all the four sides of the device to be tested. In this case the arrangement of the probes in correspondence with the device angles is to be taken into account, so as to avoid the contact with one another which would invalidate the outcome of the test performed.
It is known from the Italian patent application No. MI2001A000567 filed on Mar. 19, 2001 in the name of Technoprobe S.r.l., as well as from the corresponding European patent application No. 1 243 931 of Mar. 15, 2002 and from the corresponding U.S. patent application Ser. No. 10/102,449, now issued U.S. Pat. No. 6,768,327, to realize contact probes for testing heads provided with a rigid arm laterally projecting from a probe body, as schematically shown in FIG. 3.
In particular, the testing head 30 shown in FIG. 3 comprises an upper die 12A and a lower die 12B having respective guide holes 13A and 13B effective to house a contact probe 14.
The contact probes 14 have contact tips 15 intended for abutting on a plurality of contact pads 16 of an integrated electronic device to be tested, schematically indicated with 17.
The testing head 30 shown in FIG. 3 comprises loose-fitting probes having a further end 18 in contact with a micro-contact strip or space transformer 19.
A rigid arm 20 extends along a direction being perpendicular or sloping of a suitable angle with respect to the probe 14, i.e. it has an axis B-B being perpendicular or sloping with respect to an axis A-A of the contact probe 14, and it ends with the contact tip 15 of the probe 14 for the contact pads 16 of the integrated electronic device 17 to be tested.
In this way, the contact between the tip 15 of the probe 14 and the pad 16 is spaced of a value Z with respect to the axis A-A of the contact probe 14 itself thus allowing to reduce the value of the minimum pitch of the contact tips 15 and making it possible to test integrated electronic devices with contact pads 16 with close contact centers C, i.e. reduced pitches, Z being the distance between the axis AA and a parallel axis passing through the contact center C of the probe.
Although advantageous under several aspects, the testing head 30 realized according to the prior art has different drawbacks.
The first among all stays in the need of deforming the probe body 21 so as to realize the rigid arm 20. The method for realizing the contact probe has thus to provide a deformation step which must be very precise and repeatable, in order to guarantee the fittingness of all the probes belonging to a same testing head.
Beside adding a critical step to the method for realizing the contact probes, this deformation introduces weakness elements in the body 21 of the probe itself.
Moreover, the total space of the contact probe thus realized suffers from the longitudinal size D introduced by the rigid arm 20.